- Force of attraction among the constituting particles of matter – Greater the force of attraction more the particles pull each other and inter particle distance is lesser which means particles are closely packed and bonding among the constituting particles of matter is stronger. Stronger bonding in particles leads to stronger matter and hence such matters in which inter particle distance is lesser, particles have more force of attraction and are closely packed exists in solid state.
- If inter particle force of attraction in a matter is lesser, particles have weaker bonding among them and hence inter particle distance is more and they are not closely packed so these particles have considerable degree of freedom of movement and such matters posses fluidity so also called fluids, these exist in liquid state.
- In gases maximum inter particle distance exists, force of attraction among particles is least and particle move independently, such matters posses maximum fluidity and exist in gaseous state.
Change of state in matters
- Solid to liquid (melting) – by some means increase interparticle distance, reduce force of attraction among constituting particles, weaken the bonding to such level that particles of that matter leave their positions and start flowing with each other i.e. solid melts and changes to liquid.
(i). This change of solid to liquid is also called fusion.
(ii). Temperature at which solid matter melt to liquid at atmospheric pressure is called melting point of that matter.
(iii). Stronger the forces of attraction between constituting particles stronger the bonding and higher will be melting point.
Latent heat of fusion – when we start heating a solid its temperature keeps on rising and at some temperature solid starts melting. Now it has been noticed that temperature of solid stops rising once melting has started although we continue supplying heat. So question arises, if temperature is not rising then where this supplied heat is going now? This heat is now utilised to overcome the force of attraction between the particles of solid when solids are changing to liquid. So this continuous supply of heat does not increase temperature any further and seems hidden and hence called Latent heat of fusion (in Latin Latere means hidden). The amount of heat required to change 1 Kg of solid into liquid at atmospheric pressure without change in temperature once melting point achieved is called Latent heat of Fusion.
(d). Interesting fact – ice used for cooling a substance is more effective than water at same temperature (0oC is freezing point of ice and melting point of water so both ice and water coexist). Explanation – When ice changes to water it absorbs heat equal to latent heat of fusion and hence energy of water at zero degree Celsius is more than energy of ice at same temperature, lesser the energy more is cooling effect so ice is more effective.
(e). Latent heat of fusion of ice is 3.347 x 105 J/Kg or 80 kcal/Kg
2. Liquids to solids (freezing) – we know particles of a liquid matter have considerable degree of freedom of movement. To convert liquid to solid lower the energy of these constituting particles so that to cease their movement, attractive force among the particles bring them closer and substance freezes.
(a). Temperature at which a liquid substance freezes to become solid at atmospheric pressure is called freezing point of that substance. Process is also called solidification.
(b). Freezing point and melting point of a substance is same e.g. freezing point of water is 00 C also melting point of ice is 00
(c). If we add impurity to a liquid its freezing point decreases.
(d). If we put some weight on an ice block we will see ice melts faster where the weight is put, this shows if we increase the pressure freezing point decreases i.e. ice is normally at 0 degree Celsius and now after putting pressure freezing point is reduced so ice exists as water at this temperature.
3. Liquid to gases (boiling) – when we increase the energy of liquid particles by supplying heat energy kinetic energy of the liquid particles increases and force of attraction among the particles is considerably reduced and particles on the surface of liquid start overcoming the force of attraction and leave the surface and converted to gaseous state. If we keep on supplying the heat temperature of liquid keeps on rising and more and more liquid particles leave the surface of liquid and converted to gases, but at one temperature liquid starts boiling and now there is no further rise in temperature even if heat supply is continued. So again the question arises if the temperature is not rising where is the supplied heat energy going? The answer is that this heat energy is utilised in overcoming the force of attraction between liquid particles. This fixed temperature at which a liquid starts boiling at atmospheric pressure is called boiling point of the liquid.
(a). Latent heat of vaporisation – it is the amount of heat energy required to convert one Kg of liquid into gaseous state at atmospheric pressure at its boiling point.
(b). Latent heat of vaporisation of water is 22.59 x 105 J/Kg or 540 Kcal /Kg.
(c). Under normal condition vaporisation of water is continuous phenomenon and its rate is directly propotional to temperature.
(d). Steam at 1000 C has more energy than water at same temperature, that’s why burns from steam are more severe than burns from water at same temperature.
(e). Impurities added to liquid increase its boiling point. If we put some salt in water while boiling eggs boiling point of water increases and eggs are boiled on higher temperature and boiled better.
(f). Increase in pressure increases boiling point. Water boils in pressure cooker at much higher temperature than in open utensils, so food is cooked better and faster in pressure cooker. Also high temperature kills harmful bacteria and germs more effectively.
(g). Decrease in pressure reduces boiling point. At higher altitudes e.g. mountains atmospheric pressure is lower and hence water starts boiling at lower temperature and temperature is constant after that, this is the reason that it takes lots of time in cooking in high altitude areas.
4. Condensation – when the temperature of gases is lowered kinetic energy of the particles is reduced and particles now move slowly, on further lowering of temperature gas particles come closer and condense i.e. gases change to liquid.
This can also be achieved by applying pressure on gases in a container and restrict the molecular motion of gases and force them to come closer thus increasing inter molecular force of attraction which will make the gases to condense.
5. Sublimation – some solids on heating directly converted into gaseous state and on cooling converted to directly solid state from gaseous state without passing intermediate liquid state e.g. naphthalene, camphor, ammonium chloride, iodine etc this is known as sublimation.
Some interesting facts about inter change in states of matter:
- In advanced refrigerators nowadays during defrost cycle ice on the walls of freezer sublimes and we need not to put container to collect water like in older day’s refrigerators.
- LPG (cooking gas) – gas is compressed under very high pressure and it is converted to liquid but when it comes out slowly through the gas pipe at atmospheric pressure it is converted to gas again and is stored as liquid but burns as gas.
- Dry ice – when carbon dioxide (CO2 ) is cooled under low pressure it is converted directly to solid without passing through intermediate liquid state, also if this solid is brought at normal atmospheric pressure it sublimes without melting and hence if we touch it it’s not wet hence this is called as dry ice. Under name dricold it is used as refrigerant.
- Ether is sprayed on skin before minor surgery – ether evaporates quickly by absorbing required energy from skin in contact so skin looses heat and its temperature becomes low and area becomes numb and hence patient feels very lesser pain.
- Water droplets seen on outer surface of glass filled with chilled water – water vapours in atmosphere coming in contact of glass lose their energy and condense on the surface of glass.
- In cold countries roads are often blocked due to snowfall and it has been observed that solution of common salt (NaCl) or Calcium Chloride (CaCl2) or Magnesium Chloride (MgCl2) is sprayed on snow to melt it down, because adding these impurities lowers the melting point and snow starts melting on existing temperature. Chloride ion present in the above mentioned salts is very harmful for soil as well as aquatic organisms, as ultimately the melted snow flows to some water body and effects aquatic organisms. So it is recommended to use potassium acetate to lower the melting point of snow, acetate ion is not harmful and potassium provides nutrition.
- Perspiration causes cooling – when sweat on skin evaporates it absorbs energy from skin and temperature of skin is lowered giving cool feeling.